The hydrotreating process is a dominant process technology in refineries for fuel upgrading and clean-up. The hydrotreating reaction can be classified into four categories: hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodemetallation (HDM), and hydrodeoxygenation (HDO). In many cases, these reactions proceed simultaneously inside the reactor. Among them, HDS is of primary importance. The HDS reaction involves the breakage of carbon-sulfur bonds by addition of hydrogen molecules so as to release sulfur as H2S gas. The carbon-sulfur bonds often exist inside an aromatic molecular structure, requiring the HDS reaction to be concomitant with aromatic saturation.
Current commercial HDS technology is mature and is based on cobalt/molybdenum impregnated gamma-alumina or on nickel/molybdenum impregnated catalysts. These catalysts are employed in large reactors as random packed beds of spherical, cylindrical, or shaped extrudate beads. HDS reactors typically operate in a trickle-bed mode wherein the raw, high sulfur-containing distillate-range hydrocarbon liquid flows at relatively low velocity downward through the catalyst bed, while a hydrogen-rich gas flows co-currently downward through the catalyst at a much higher velocity. Organic sulfur compounds and organic nitrogen compounds in the distillate are converted to hydrogen sulfide and ammonia, which are separated from the treated liquid product downstream in the vapor/liquid separator and in a stripper distillation tower. The sour gas is often treated by amine absorption to remove the H2S and NH3, with the large excess of hydrogen containing gas recycled back to the process. The recycle gas rate is typically adjusted so as to provide an excess of hydrogen over the stoichiometric requirements for reaction. This results in the hydrogenation of olefins and other desirable compounds that may be present in the hydrocarbon feed or generated during the hydrotreating process.
Cracked naphtha is a blending component commonly used in refinery gasoline pools. Cracked naphtha can be produced in refinery fluid catalytic cracking processes, coking processes, or hydrocracking processes, among others, where a gasoline boiling range component is generated or distilled having olefinic compounds. Cracked naphtha typically contains both sulfur and olefin compounds. The sulfur compounds, which can be present in cracked naphtha in amounts ranging up to 1.0 percent by weight, are potential air pollutants and a poison to the catalysts used in automobile catalytic converters. Thus it is desirable to remove the sulfur compounds from the cracked naphtha. However, desirable olefin compounds can be present in cracked naphtha in an amount ranging up to 60 percent by weight. The olefin compounds are desirable because they have octane numbers that are generally higher than the octane numbers of their corresponding saturates. Thus, it is generally undesirable to saturate olefins to saturates wherein the component stream is to be blended directly to gasoline. If the cracked naphtha is to be desulfurized without eliminating or seriously reducing the olefin content, the hydrodesulfurization process needs to be very selective. That is to say the hydrodesulfurization process should remove substantially all of the sulfur compounds without severely saturating the olefins that are present.
Currently, there are several hydrotreating catalysts and processes that find considerable use in the petroleum refining industry. Such hydrotreating catalysts include a variety of transition metals such as cobalt and molybdenum and their compounds on a suitable support, cobalt, molybdenum, and nickel on a suitable support, nickel and tungsten and compounds thereof on a suitable support, and nickel and molybdenum and compounds thereof on a suitable support. The support, in general, is a weakly-acidic catalytically active alumina. Typically the alumina support is in pellet form and formed as extrudate of a slurry of alumina powder. These extruded alumina pellets are then impregnated with solutions containing precursors and then calcined and sulfided prior to use. Such conventional hydrodesulfurization catalysts are less selective and not only remove sulfur from the petroleum hydrocarbon stream being treated, but also tend to saturate olefins, reducing the octane of the petroleum hydrocarbon stream.
Research over the last couple of decades has resulted in a great many hydrodesulfurization catalysts and processes for desulfurizing naphtha feed streams, while attempting to keep olefin saturation at a minimum. While there are commercially successful naphtha hydrodesulfurization catalysts in use today, there is a continuing need for improved catalysts, methods and reactors that are capable of combining optimum hydrodesulfurization with minimum hydrogenation of olefin.
The present invention includes a process for the hydrotreating of a hydrocarbon feed. Such an illustrative process includes reacting the hydrocarbon feed and a hydrogen containing gas in a reactor containing a monolithic honeycomb catalyst bed to give a hydrotreated hydrocarbon product. The monolithic honeycomb catalyst bed is formulated to include a monolithic honeycomb refractory support and a suitable hydrotreating catalyst. In carrying out the illustrative process, the hydrogen containing gas to hydrocarbon feed liquid volume ratio is preferably greater than about 10 NL/L, the liquid hourly space velocity is preferably greater than about 1 liter feed per hour per liter of catalyst bed volume, the reactor pressure is preferably greater than about 50 psig, and the monolithic honeycomb catalyst bed temperature is preferably greater than about 50xc2x0 C. In one illustrative embodiment, the physical feature of the monolithic honeycomb catalyst bed are such that it has a channel density of about 25 to 1600 cells per square inch; a channel size from about 0.1 to 10 mm; and a channel wall thickness of about 0.01 to about 2.0 mm.
The hydrotreating catalyst components include a powdered refractory oxide and transition metal catalyst compounds or alternatively the hydrotreating catalyst components are impregnated into the monolithic honeycomb refractory support itself. Preferably the hydrotreating catalyst components include a Group VIII metal containing compound and the Group VIB metal containing compound. In one illustrative embodiment, the Group VIII metal is cobalt and the Group VIB metal is molybdenum. When the Group VIB metal is molybdenum, the molybdenum content of the hydrotreating catalyst components measured as weight percent of MoO3 is preferably from about 12 to about 18. Similarly when the Group VIII metal is cobalt, the cobalt content of the hydrotreating catalyst components measured as weight percent of Co3O4 is preferably from about 2 to about 5. In addition to catalytic transition metal components, and alternative illustrative embodiment of the hydrotreating catalyst components may also include a phosphorous promoter. In such instances the phosphorous promoter content measured as weight percent of P2O4 is preferably from about 0.1 to about 2. Irrespective of the method of formulating the monolithic honeycomb catalyst bed, the hydrotreating catalyst components are preferably from about 1 weight percent to about 100 weight percent of the monolithic honeycomb catalyst bed. One illustrative and preferred embodiment of the present invention involves the hydrodesulfurization of cracked naphtha.
The present invention also encompasses a reactor for the hydrotreating of hydrocarbon feedstocks. Such hydrocarbon feedstocks include for example, naphtha fractions, heavy cracked naphtha, and other similar hydrocarbon feeds in which hydrotreating is desired. One illustrative embodiment of the reactor includes a reaction chamber having a feed inlet and a product outlet; and a monolithic honeycomb catalyst bed inside the reaction chamber. The monolithic honeycomb catalyst bed may be composed of one or more modules that are composed of a monolithic honeycomb refractory support and a hydrotreating catalyst component.